In conventional digital communications systems, a sender uses a message signal to transmit message symbols to a receiver. The sender and receiver agree upon a transmission scheme such that the mapping between signals and symbols are unique and known by both parties. In order to satisfy requirements for stealth, robustness, and security of a communication system, authentication, integrity, and secrecy of the signal transmission via a transmitting media is to be provided. For an authentication system, uniqueness and non-reproducibility of the identification signal are of the utmost importance.
Research in authentication techniques have focused mostly above the Physical Layer (PHY) of the Open System Interconnection (OSI) model underlying the operation of the network system. As is known to those skilled in the art, the OSI model is an abstract description for layered communications and computer network protocol design. The OSI divides network architecture into seven layers, out of which the Physical Layer (PHY) is the bottom layer. The function of the PHY is to define the electrical and physical specifications of a device, and, in particular, to define the relationship between the device and a physical medium, including performing encoding and signaling functions that transform data from bits residing within a device into signals to be sent over the network. The PHY also defines specifications as to data transmission and reception at the device.
There are two paradigms conventionally used in communication systems for adding authentication: multiplexing or embedding. Examples of multiplexed authentication may be represented by message authentication codes or authentication protocols that require a series of message devoted to authentication. An overview of these methods may be found in G. J. Simmons, “A survey of information authentication”, Proceedings of the IEEE, Volume 76, Issue 5, May 1988, pp. 603-620; as well as in Chapters 9 and 10 of A. J. Menezes, P. C. van Oorschot, and S. A. Vanstone, “Handbook of Applied Cryptography”, 5th printing, CRC Press, 2001. The advantage of these methods is that the authentication is received with the same quality as the data. However, data throughput is penalized since some of the bits carry authentication instead of data.
In 1972, T. Cover, “Broadcast channels”, IEEE Transactions on Information Theory, Volume 18, Issue 1, January 1972, pp. 2-14 analyzed broadcast channels and demonstrated that high joint rates of transmissions are best achieved with simultaneous, as opposed to time-multiplexed, transmissions. Digital watermarking follows the paradigm of embedded signaling by modifying the data in a controlled manner that provides additional information to the receiver. Authentication may be transmitted in this manner as presented in C. Fei, D. Kundur, and R. H. Kwong, “Analysis and Design of Secure Watermark-based Authentication Systems”, IEEE Transactions on Information Forensics and Security, Volume 1, No. 1, March 2006, pp. 43-55; as well as in L. M. Marvel, C. G. Boncelet, and C. T. Retter, “Spread Spectrum Image Steganography”, IEEE Transactions on Image Processing, Volume 8, Issue 8, August 1999, pp. 1075-1083. The embedded signaling for adding the authentication has proven to provide stealthy authentication. However, as opposed to the multiplexing approach, embedding of additional information degrades the data quality (I. J. Cox, M. L. Miller, and A. L. McKellips, “Watermarking as Communications with Side Information”, Proceedings of the IEEE, Volume 87, Issue 7, July 1999, pp. 1127-1141). Much of the research in digital watermarking has focused on watermarking multimedia data and minimizing the distortion at the receiver in terms of human perception.
At the Physical Layer, work has been done in authenticating the sender and receiver based on prior coordination or secret sharing, where the sender is authenticated if the receiver can successfully demodulate and decode the transmission. Spread spectrum techniques, such as direct sequence and frequency hopping, may be viewed as examples of physical layer authentication systems (J. G. Proakis, Digital Communications, 4th ed. New York: McGraw-Hill, 29000, Chapters 5, 13). While these techniques are covert and provide robustness to interference, they achieve this at the cost of bandwidth expansion. Additionally, if it is desired to add authentication to a system in a stealthy way so that users unaware of the authentication continue to communicate without modifications to hardware or protocol, the technique does not serve this purpose well, since only authenticated parties with knowledge of the secret are allowed to participate in communications. The need for such stealth arises, for example, when authentication is piggybacked onto an existing system.
The idea of transparently adding information at the physical layer has been studied for some specific cases. S. H. Supangkat, T. Eric, and A. S. Pamuji, “A public key signature for authentication in telephone”, APCCAS 2002, Volume 2, pp. 495-498 proposed one such authentication scheme for telephony where an encrypted hash of the conversation is added back into the signal. Similarly, J. E. Kleider, S. Gifford, S. Chuprum, and B. Fette, “Radio Frequency Watermarking for OFDM Wireless Networks”, ICASSP 2004, Volume 5, pp. 397-400 proposed a scheme where a low-power watermark signal is added to the data signal with spread spectrum techniques. X. Wang, Y. Wu, and B. Caron, “Transmitter identification using embedded pseudo random sequences”, IEEE Transactions on Broadcasting, Volume 50, Issue 3, September 2004, pp. 244-252 proposed a scheme for broadcast television where each transmitter adds a unique low-power signal to its transmission in order to prove its identity to the receivers.
The transparent transmission of data may also be built by using multi-resolution transmission, where varying levels of protection are guaranteed for multiple data streams as presented in L. F. Wei, “Coded modulation with unequal error protection”, IEEE Transactions on Communications, Volume 41, Issue 10, October 1993, pp. 1439-1449; P. K. Vitthaladevuni and M. S. Alouini, “Exact BER computations of generalized hierarchical PSK constellations:”, IEEE Transactions on Communications, Volume 51, Issue 12, December 2003, pp. 2030-2037; and M. Morimoto, M. Okada, and S. Komaki, “A hierarchical image transmission system in a fading channel”, Fourth IEEE International Conference on Universal Personal Communications, November 1995, pp. 769-772. With this scheme, data symbols are sent at high rate while the authentication is sent at a lower rate. Multi-resolution (also known as asymmetric or nonuniform) constellations provide important data signal points to be far apart while less important signal points are close together.
Authentication at the physical layer may be viewed as a special use of pilot symbols inserted in the transmitted signal, since the authentication signal is verified and therefore known at the receiver. However, a subtle difference arises since the authentication signal may or may not be present in the received signal. Pilot symbols are either time division multiplexed (TDM) or superimposed (SI) with the transmitted messages. M. Dong, L. Tong, B. M. Sadler, “Optimal insertion of pilot symbols for transmission over time-varying flat fading channels” IEEE Transactions on Signal Processing, Volume 52, Issue 5, May 2004, pp. 1403-1418 showed that SI schemes may outperform TDM schemes when the transmission channel becomes sufficiently time varying. For a packet-based multi-carrier system, J. E. Kleider, G. Maalouli, S. Gifford, S. Chuprun, “Preamble and embedded synchronization for RF carrier frequency-hopped OFDM”, IEEE Journal on Selected Areas in Communications, Volume 23, Issue 5, May 2005, pp. 920-931 suggested that SI pilot symbols may be used for channel acquisition while incurring only a 1 dB penalty when compared to a TDM training scheme.
Although a vast amount of research has been performed in the field of authenticated communication there still is a need to improve stealth, robustness and security of authentication schemes by hiding the authentication in the physical waveform while maintaining high levels of robustness and security.